What is the proper formula to use for calculating the amount of electric heat needed in a room the size of: 36'x24'x10'? In the past I've used 1.5 watts per cubic ft. Am I close?

Don't know how cold it gets where you are.
For Colorado Mountains I used 10 watts per square foot with 8' ceilings.

No simple rule of thumb. Outside temperatures vary, wall insulation varies, window area and type varies, how many walls, floors, ceilings have outside exposure and how many adjoin other heated space, what's the wind like, how much solar heat gain do you get, etc.

The good news is with electric heat, if you oversize it doesn't hurt anything or cost anymore to operate it just operates for shorter cycles.